This invention relates to xerographic systems and, more particularly, to the development of latent images in touchdown development systems.
The xerographic process as disclosed in Carlson's U.S. Pat. No. 2,297,691, encompasses a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing. This plate is provided with a uniform electric charge over its surface and is then exposed to a light image representing the subject matter to be reproduced. The light exposure discharges the plate areas in accordance with the light radiation intensity that reaches it and thereby creates a latent, electrostatically charged image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged finely divided material, such as an electroscopic powder called toner, that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a selective pattern corresponding to the latent electrostatic image. Thereafter, the developed image may be fixed by any suitable means to the surface on which it has been developed or the developed image may be transferred to a secondary support surface to which it may be fixed or utilized by means known in the art.
Once the electrostatic image is formed, the method by which it is made visible is the developing process. Various developing systems are well known in the art and include cascade, brush development, magnetic brush, powder cloud and liquid development. Still another developing method is disclosed in Mayo, U.S. Pat. No. 2,895,847 in which a support member, called a "donor," is employed to present a releasable layer of electroscopic (toner) particles to the photoconductive layer for deposit thereon in conformity with the electrostatic image. The Mayo approach is one of several variations which involve the transfer of toner particles from a donor to the photoconductive surface and is therefore called transfer development. This technique is also known as "touchdown development."
Efforts have been made in the past to provide flexibility in a xerographic machine to change from a positive-to-positive (direct) or positive-to-negative (reversal) developing process. These approaches included systems with touchdown development or other development processes. This choice of development polarity is called ambipolar development.
Of the several approaches to ambipolar development now in use, none is fully satisfactory for a variety of reasons. One process of development employs what is called a reversal developer which is used on the Model 1824 xerographic machine made by the Xerox Corporation. In this machine, a change in the mode of development (e.g., positive-to-positive to negative-to-positive) is effected by replacing the entire developer.
Another approach to ambipolar development employs a non-standard photoreceptor which is photoconductive in either a positive or negative charged mode. (Standard selenium photoreceptors are usually photoconductive only in the positively charged mode).
Another ambipolar developer employed the use of positive and reversal coated carrier beads: such a system is limited to line copy systems only because of difficulties in the development of solid area images.
There has therefore been a need for a provision in xerographic machines whereby clear solid area and line image can be effected together with ambipolar development without major modification to the machine or without using sophisticated or non-standard components.